As the scaling rule, known from the Moor's law, is becoming more miniscule, the integration degree of semiconductor devices, including silicon devices, is becoming progressively higher, while their power consumption is becoming progressively lower. The semiconductor devices have been developed at a pace of four times in three years in terms of the degree of integration. Recently, the gate length of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is 20 nm or less. Due to rise in the cost of the lithographic process and physical limitation placed on the device size, a need is felt to improve the device performance by an approach different from the conventional approach founded on the scaling rule.
The rise in the cost of the lithographic process may be said to be attributable to rise in the costs of manufacturing apparatus and the mask set. Among the physical limitations in the device size, there are the limitations in the operation and the limitations in the dimensional variations.
Recently, a rewritable programmable logic device, termed FPGA, has been developed. FPGA is regarded to be at an intermediate position between a gate array and a standard cell. The FPGA allows a customer to configure a circuit as he/she desires after manufacture of the device. The FPGA includes a variable resistance element in the inside of a multilevel interconnection structure to allow the customer to make electrical connections of interconnects as he/she desires. The circuit may be improved in the degree of freedom with the use of the semiconductor device carrying the FPGA. The variable resistance element may be enumerated by ReRAM (Resistance RAM (Random Access Memory)) and NanoBridge (registered trademark of NEC Corporation, the assignee company) that uses an ion conductor. It is noted that the ion conductor is a solid substance in which ions may move freely on application of, for example, an electrical field.
Non-Patent Literature 1 discloses, as a variable resistance element that is likely to improve the degree of freedom of an electrical circuit, a switching element making use of an electrochemical reaction and metal ion mobility in the ion conductor. The switching element disclosed in Non-Patent Literature 1 is composed by three layers, viz., an ion conduction layer and first and second electrodes provided in contact with two sides of the ion conduction layer. It is noted that, while the first electrode performs the role of supplying metal ions to the ion conduction layer, there are no metal ions supplied from the second electrode.
The operation of the switching element will now be briefly described. When the first electrode is grounded and a negative voltage is applied to the second electrode, metals of the first electrode are turned into metal ions and are dissolved in the ion conduction layer. The metal ions in the ion conduction layer are precipitated as metal in the ion conduction layer. The so precipitated metal forms metal crosslink(s) that interconnects the first and second electrodes. With the metal crosslink(s) electrically interconnecting the first and second electrodes, the switch is turned to an on-state.
Conversely, if, in the above mentioned on-state, the first electrode is grounded and the positive voltage is applied to the second electrode, part of the metal crosslink(s) is severed. This breaks the electrical connection between the first and second electrodes, so that the switch is turned off. Note that, as from the stage before complete breakage of the electrical connection, the electrical characteristic is changed. For example, the resistance between the first and second electrodes increases, or the capacity between the electrodes is varied. Ultimately, the electrical connection is turned to an off-state. In case the on-state is to be re-established from such off-state, it is sufficient that the first electrode again is grounded and a negative voltage is applied to the second electrode.
Non-Patent Literature 2 discloses a construction and an operation of a two-terminal switching element in which two electrodes are arranged via an ion conductor and which controls the state of conduction through these electrodes.
These switching elements feature a size and an on-resistance smaller than those of the semiconductor switch, such as MOSFET, and hence the application of the switching elements to a programmable logic device is felt to be promising. In addition, in these switching elements, the state of conduction (on- or off-state of the elements) may be maintained as it is without voltage application. Hence, the switching elements may possibly be used as non-volatile memory devices.
For example, a memory cell composed by a transistor selection element and a transistor switching element is arranged as a basic unit to form a volatile memory. A plurality of the memory cells is arranged in each of the longitudinal and transverse directions to form a plurality of word lines and a plurality of bit lines. Arbitrary one of the memory cells may be selected by these word lines and bit lines. The state of conduction of the switching element of the selected memory cell is sensed. From the on/off state of the switching element, it is possible to read out which of the information ‘1’ and the information ‘0’ is stored in the so selected memory cell.
In the Patent Literatures 1-3, there are also disclosed nano-channel switching elements or memory elements.
PTL 1:
    WO2007/114099PTL 2:    JP Patent Kokai Publication No. JP2006-222428APTL 3:    JP Patent Kokai Publication No. JP2006-261677ANPL 1:    Shunichi Kaerimiya et al., “A Nonvolatile Programmable Solid-Electrolyte Nanometer Switch”, IEEE Journal of Solid-State Circuits, Vol. 40, No. 1, pp. 168-176, January 2005Non-Patent Literature 2:    M. Tada, T. Sakamoto, Y. Tsuji, N. Banno, Y. Saito, Y. Yabe, S. Ishida, M. Terai, S. Kotsuji, N. Iguchi, M. Aono, H. Hada and N. Kasai, “Highly Scalable Nonvolatile TiOx/TaSiOy Solid-electrolyte Crossbar Switch Integrated in Local Interconnect for Low Power Reconfigurable Logic”, IEEE International Electron Devices Meeting (2009, Baltimore, USA), pp. 943-946 (2009)